The invention relates to a small domestic appliance driven by means of a single-phase synchronous motor, whose drive shaft drives a load whose work function is not dependent on the direction of rotation, the noise and vibrations produced by the motor being dependent on the direction of rotation of the motor.
Small domestic appliances whose work function is not dependent upon the direction of rotation of the motor are known. Such appliances include all appliances whose working movement is a reciprocating movement, as for example electric shavers with reciprocating cutters or hair trimmers. These reciprocating movements of the cutter means can be obtained by means of an oscillating armature motor or by means of an electric motor with a revolving armature, for example a single-phase synchronous motor. Such a drive for a hair-cutting appliance is described in, for example, the magazine ETZ, Vol. 30, 1978, H2, pages 56 to 60.
Also known are other small domestic appliances, for example citrus-juice extractors or knife sharpeners, which perform a rotary working movement and in which the tools perform their work functions independently of the direction of rotation. Moreover, it is known to drive such small domestic appliances with rotary tools by means of single-phase synchronous motors.
In small domestic appliances driven by means of single-phase synchronous motors, the dependence of the operating noise on the direction of rotation of the motor may be perceptible to a smaller or greater extent. In some cases only the pitch of the noise differs. However, there are also cases in which the noise in one direction of rotation is substantially louder and more unpleasant than in the other direction. This may disturb or annoy the user, inducing him to a negative opinion, or even complete rejection of the appliance.
The difference in noise may be caused by a motor characteristic which is dependent on the direction of rotation. Allowance is to be made in particular for asymmetries in the conversion of rotary movements into oscillating movements.